Add motif creation/decoding

[steplica]

This will add an endpoint, /download/motif which can download a custom motif file from a JSON payload of the following format:

{
    "pdbId": "1yph",
    "ecNumber": "3.4.14.21",
    "activeSiteResidues": [
        {
            "residueName": "His",
            "residueChainName": "C",
            "residueId": "57"
        },
        {
            "residueName": "Asp",
            "residueChainName": "C",
            "residueId": "102"
        },
        {
            "residueName": "Gly",
            "residueChainName": "E",
            "residueId": "193"
        },
        {
            "residueName": "Ser",
            "residueChainName": "E",
            "residueId": "195"
        },
        {
            "residueName": "Gly",
            "residueChainName": "E",
            "residueId": "196"
        }
    ]
}

It also decodes these motif files when they're received by the backend.